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There's this: https://wordpress.org/plugins/wp-smushit/ but it looks like that the online service it was using is no more available.
There are also plugins like...

There's nothing to do, PHP scripts can't use the exec function. You have to use a plugin that optimizes images using an external service.

Did you wait some minutes? The unblock isn't immediate.
If you still read error messages, could you please write the exact steps needed to reproduce the problem?

Did you unblock the server to server connections? You can do it from the AlterVista control panel: go to the Resources section, then click on Edit PHP Settings. In that page you can find the...